1. Field of the Invention
The present invention relates to the production of alumina-based catalyst carriers (supports) having improved durability, and, more especially, to the production of such alumina-based carriers, or supports, in the form of alumina agglomerates.
2. Description of the Prior Art
It is well known to this art that, for numerous applications, such as the treatment of exhaust gases emanating from internal combustion engines, the hydrotreatment of petroleum products, dehydrohalogenation, hydrocracking, hydrogenation, dehydrogenation, dehydrocyclization of hydrocarbons or other organic compounds, oxidation reactions and reduction reactions, catalysts are used, the carrier therefor being alumina-based.
It is also well known to this art that the efficiency of a catalyst is generally the greater, the larger the contact area between the catalyst and the reactants. To achieve this, it is necessary that the catalyst should be maintained in as finely divided a state as possible, that is to say, the solid particles comprising same should be as small and individualized as possible. The fundamental role of the carrier, hence, is to maintain the catalyst particles or crystallites in contact with the reactants, in as finely divided a state as possible.
Of course, part of the concept of a carrier is that the latter must be able to physically maintain the catalyst crystallites in position in the reactor where the chemical reaction takes place, and it is for this reason that the carrier must have a mechanical strength appropriate to the selected application.
The quality of a catalyst carrier is thus firstly linked to its specific surface area (SBE), which can be determined by the BET method and, secondly, to its mechanical strength (EGG), which can be determined by measuring the force required to crush a carrier particle.
Nevertheless, this characterization is inadequate because it is important to the user not only that the fresh catalyst should be very active and firm, but that it should remain so for as long a period of time as possible, such as to reduce the costs involved in replacing the catalyst bed.
It will thus be seen that it is necessary to take into account a durability criterion when evaluating a catalyst carrier.
Given the role of the carrier in heterogeneous catalysis, the durability of a carrier can be defined as the ability to retain a high surface area in the reactor in which it is employed, after a certain period of exposure to the thermal and mechanical conditions prevailing in the reactor.
Indeed, two superposed factors are involved in the aging of a catalyst carrier:
First, irreversible phase changes can occur, especially under the influence of high temperatures, causing sintering, namely, a reduction in the specific surface area due to the coalescence of the very fine micropores. During this coalescence, a portion of the catalyst is engulfed in the carrier mass and is no longer in contact with the reactants; and
Second, and this phenomenon is particularly accentuated in the case of bubbling beds or fluidized beds and also in automobile post-combustion treatment, the impacts between the agglomerates can result in a surface abrasion which produces fine particles which can be entrained in the stream of reactants. Thus, this contributes to a reduction in catalyst activity due to a loss of catalyst in the reactor.